Leptin administration induces weight loss in rodents via its effects on food intake and energy expenditure. High carbohydrate/low fat (HC/LF) diets induce weight loss in humans, but the mechanism is not well understood. In humans, there is a nocturnal increase of plasma leptin concentrations which is proportional to insulin responses to meals, but this increase does not occur when subjects are fasted. We have recently reported that adipocyte glucose uptake and metabolism are important determinants of insulin-induced leptin secretion in vitro. Accordingly, HC/LF meals, which induce greater excursions of circulating insulin and glucose, should produce higher leptin concentrations than low carbohydrate/high fat (LC/HF) meals. Women ranging in body mass index from 20-29 kg/m2 will be tested under two dietary conditions; the high carbohydrate diet composed of three meals, each containing 730 kcal (60% carbohydrate, 20% fat, 20% protein) or the high fat diet composed of three meals each containing 730 kcal (60%fat, 20% carbohydrate, 20% protein). Blood samples will be taken over a 24h period at 30-60 min intervals.